1. Field of the Invention
The invention relates to enhancing the antibody response to a target antigen, by incorporating the antigen into a liposome along with an additional constituent which contains at least one T-helper lymphocyte recognition site.
2. Description of the Related Technology
A vaccine antigen confers immunity to infection by inducing the organism to produce an immune response. Antigens are, therefore, of varying usefulness depending on the strength of the immune response they induce. There are to varying degrees inherently strong and weak antigens. Additionally, antigens can be classified as thymic ("T") dependent or T-independent antigens based on whether they are capable of eliciting helper activity from T-cells. This T-cell activity is associated with the production of antibodies of the IgG and IgA classes. T-independent antigens induce the production of IgM antibodies, but do not induce the B-cells to switch to the synthesis of IgG or IgA antibodies. The production of immunoglobulins of the IgM class is a transient response in laboratory animals and humans, lasting only a few months; whereas the production of antibodies of the IgG and IgA classes usually persist for years. It is, therefore, beneficial to elicit a T-dependent response to a particular antigen. Carbohydrate or polysaccharide based antigens are T-independent antigens and are of limited usefulness as vaccines in children. Similarly, many polypeptides which represent antibody recognition sites are T-independent antigens and are, therefore, likewise incapable of generating the IgG and IgA antibody responses that are desired for a vaccine.
It is known that a stronger antibody response can be elicited to a weak antigen by conjugating (covalently attaching) the antigen to a helper protein which enhances the immune response. For example, in U.S. Pat. No. 4,761,283 it was shown that the weak immunogenic response to certain bacterial capsular polymers was enhanced by conjugating the antigen to a bacterial helper protein which, in itself, induces an antigenic response. It is common to use a toxin as the helper protein, such as for example, Diphtheria Toxoid. Use of such a toxic helper protein creates a problem, however because the intrinsic toxicity of the helper protein may limit the dosage of the antigen-helper protein conjugate and therefore limit its effectiveness.
It is also common that the immune response to the target antigen is suppressed due to a previous immunization of the organism to the helper protein creating an epitope suppression effect. When the host organism is primed to react to the helper protein, the organism will clear the body of the target antigen-helper protein conjugate before the organism can initiate an immunological response to the target antigen.
These previously used conjugates are formed by covalently linking the antigen to the carrier helper protein. The immune response enhancement is of particular importance for T-independent antigens which can be conjugated to a peptide containing at least one T-helper cell recognition site, thereby obtaining a T-dependent response to a T-independent antigen. The presently used covalently linked conjugates are, however, limited in their effectiveness because of the structural limitations imposed by the covalent binding process, i.e., conformational changes, potential inaccessibility of binding sites, and inability to vary the ratios of the components. In addition, these conjugates may exhibit dose limitations and an epitope suppression effect.
Liposomes are membranous vesicles formed by the dispersion of lipids in aqueous media. Methods for the preparation of liposomes are well known to those skilled in the art, and are exemplified but not limited to any of the following patents which are incorporated herein by reference: U.S. Pat. Nos. 4,565,696 and 4,235,871. Liposomes possess several properties required for an in-vivo carrier; low toxicity, low immunogenicity, and biodegradability. It has also been shown that liposomes can enhance the antibody response to antigens in laboratory animals. The antigens are either entrapped within the aqueous compartments of the liposome or associated with the bilayer. For example, U.S. Pat. No. 4,565,696 describes a process for linking immunogens covalently to the surface bilayer of the liposome and thereby potentiating the immune response.